How Do I Calculate a Percentage of a Number?
Calculating percentages is one of the most common mathematical tasks used in daily life, whether you are calculating a tip at a restaurant, figuring out a discount while shopping, or analyzing data at work. At its core, "percent" means "per one hundred."
The Standard Percentage Formula
To find the percentage of a specific number, you use the following formula:
Result = (Percentage / 100) × Number
Step-by-Step Example
Suppose you want to calculate 15% of 80.
- Convert the percentage to a decimal: 15 ÷ 100 = 0.15
- Multiply by the original number: 0.15 × 80 = 12
- Result: 15% of 80 is 12.
How to Calculate Percentage Increase or Decrease
To find out by what percentage a number has changed, you need to find the difference between the new and old numbers first.
% Change = ((New Value – Old Value) / Old Value) × 100
For example, if a price moves from 100 to 125, the increase is 25%. If it moves from 100 to 75, the decrease is 25%.
Common Real-World Applications
- Sales Tax: If your local sales tax is 8%, multiply your total bill by 0.08 to find the tax amount.
- Discounts: If an item is 30% off, calculate 30% of the price and subtract it from the original.
- Statistics: Calculating the percentage of a population that shares a specific trait.
function calculateSimplePercentage() {
var pct = parseFloat(document.getElementById('pct_input_val').value);
var total = parseFloat(document.getElementById('total_val').value);
var resultElement = document.getElementById('simple_result');
if (isNaN(pct) || isNaN(total)) {
resultElement.style.display = 'block';
resultElement.style.backgroundColor = '#ffebee';
resultElement.style.color = '#c62828';
resultElement.innerHTML = 'Please enter valid numbers in both fields.';
return;
}
var result = (pct / 100) * total;
// Limit to 4 decimal places if necessary
if (result % 1 !== 0) {
result = result.toFixed(2);
}
resultElement.style.display = 'block';
resultElement.style.backgroundColor = '#e8f0fe';
resultElement.style.color = '#1967d2';
resultElement.innerHTML = pct + '% of ' + total + ' is ' + result;
}
function calculatePercentChange() {
var start = parseFloat(document.getElementById('start_val').value);
var end = parseFloat(document.getElementById('end_val').value);
var resultElement = document.getElementById('change_result');
if (isNaN(start) || isNaN(end)) {
resultElement.style.display = 'block';
resultElement.style.backgroundColor = '#ffebee';
resultElement.style.color = '#c62828';
resultElement.innerHTML = 'Please enter valid numbers.';
return;
}
if (start === 0) {
resultElement.style.display = 'block';
resultElement.innerHTML = 'Cannot calculate change from zero.';
return;
}
var change = ((end – start) / Math.abs(start)) * 100;
var type = change >= 0 ? 'increase' : 'decrease';
resultElement.style.display = 'block';
resultElement.style.backgroundColor = '#e8f5e9';
resultElement.style.color = '#2e7d32';
resultElement.innerHTML = 'Result: ' + Math.abs(change).toFixed(2) + '% ' + type;
}